1. Field of the Invention
This invention relates to materials suitable for making cores for casting directionally solidified eutectic and superalloy materials.
2. Description of the Prior Art
Jet engines are a multi-million dollar business. The maximum operating temperature of these engines must be increased to attain higher efficiencies. Higher operating temperatures require the fabrication of new alloys with high temperature strength, toughness and corrosion resistance.
The most promising method of fabricating the turbine blade components with improved high temperature properties is by directional solidification (DS). The rate of production of blades by the DS process is partly governed by the liquidus temperature which should be as high as possible. The liquidus temperature of many of the promising alloy compositions being explored at this time is greater than 1500.degree. C. and reaches nearly 1800.degree. C. or higher. In order to make "hollowed" blades (which are mandatory for use in jet engines), the alloy is solidified around a ceramic core material which is subsequently leached away, leaving behind the proper air cooling cavities in the blade. Thus far, fused silica is the standard core material used at temperatures up to about 1550.degree. C. without deleterious reaction with many of the alloy compositions. At temperatures exceeding about 1550.degree. C., silica cores are severely attached by one or more of the most reactive elements (A1, Hf, C) of the alloy. Consequently, silica cores cannot be used at the high temperatures required to directionally solidify the alloy. MgO and Y.sub.2 O.sub.3 cores are found to react only slightly with the aluminum in the alloy during DS at temperatures near 1800.degree. C., but both materials have relatively poor leachabilities.
Although leachability and nonreactivity with the alloy up to 1800.degree. C. are the two primary characteristics of the core material, other desirable characteristics are that it (1) be economical, (2) not undergo more than an overall dimensional change of .about.2-4%, (3) have a porosity of from about 25% to about 60% to aid degassing during DS and increase the rate of leachability, (4) exhibit a modulus of rupture of only about 100 psi so that good crushability of the core occurs after the metal is cast, and (5) have good thermal shock resistance.
Considerable data compiled to data on the reactivity between ceramic core materials and nickel-base superalloy materials reveals that the major chemical reaction that occurs is between the ceramic and alumina (Al.sub.2 O.sub.3). The alumina comes primarily from the oxidation of some of the aluminum which is present in the superalloy composition. Since casting of the superalloy materials is practiced up to about 1800.degree. C., the eutectic temperature of the ceramic compound and the alumina (Al.sub.2 O.sub.3) should be higher than 1800.degree. C.
It is therefore an object of this invention to provide a new and improved material composition for making ceramic cores for directional solidification of eutectic alloy and superalloy materials.
Another object of this invention is to provide a new and improved material composition for making ceramic cores which is also economical for use in casting directionally solidified eutectic alloys and superalloy materials.
A further object of this invention is to provide a new and improved material composition for making ceramic cores having increased porosity, leachability and crushability characteristics.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.